The effluent from a waste water treatment plant contains suspended solids and may, in most instances, be regarded as a colloidal suspension of such solids. This effluent, unless treated to an 85% removal of said suspended solids by other treatment means, is not suitable for release into the environment, as a minimum condition. Depending on the receiving stream, the percentage of treatment required could go as high as 95% removal of suspended solids.
When treating the effluent from a waste water treatment plant using chemical additives to coagulate and flocculate said suspended and dissolved solids, and in conjunction with the unique apparatus for use in a sequential batch treatment to separate the solids and liquid, 100% of the solids can be removed from the effluent of a waste water treatment plant.
When water treatment consists of the sterilization only, such as with the use of disinfectant, chlorine, and the liquid contains no suspended or dissolved solids, a continuous treatment process is most effective.
However, when the water has dissolved impurities which will precipitate and/or colloidal solids in suspension, a continuous method of treatment is not cost effective. In such a situation, if an additive or coagulating or flocculating agent is necessary, the cost of treatment can increase as much as five times.
The increased cost in this situation is related to the requirement for additional phases of treatment. Such additions are (a) chemicals, (b) chemical feeders, (c) mixers to coagulate or flocculate, (d) settling chambers and (e) filters. As noted above, the continuous treatment in such a situation is not cost effective.
In the treatment of water using additives on a continuous basis, the coagulated floc created by the additives that settle, when slightly disturbed, will go back into suspension.
In the prior art this condition makes it necessary to use very large settling chambers (item "d" above), which are only 90% effective at best. Therefore, large gravity sand filters (item "c" above) must be used to remove the 10% that escaped from the settling chambers.
Both the settling chamber and the gravity filters require a great deal of space and are very expensive. In addition to the initial cost, the gravity filters use 20% of the already treated water to backwash the filters.
The cost effective means for treating such volumes of water then is by a batch process, using the same chamber for coagulating, flocculating, and settling, whereby the solids settle to the bottom of the chamber for removal and the liquid is decanted from the top.
It is essential in such a settling chamber that the clear water is decanted from the top of the chamber without disturbing the settled solids therebelow and that the settled solids be removed from the chamber when they reach a desired level, quickly and efficiently, with a minimal use of clear treated water escaping with the solids being removed.
As will be subsequently explained then in a water purification system wherein solids are to be removed, a modular plant using sequential batch processing can be much more efficient than continuous processing and the large settling tanks normally associated with water purification can be eliminated. Combining the adding of chemical to coagulate, flocculate, and settling in a single tank, however, requires both an efficient and a non-invasive means for removing the treated liquid without disturbing the settled solids, and for removing the settled solids without using undue quantities of the already treated clear water, so that once such a combination tank has processed one batch, it can be cleared for re-use.
Decanting on a continuous treatment process served no useful purpose that can not be better solved using a properly designed baffled tank and a properly designed over-flow weir. Decanting in a continuous treatment process is employed to increase the density of the settled sludge and lower the moisture content before solid disposal, such as in digesters. The decanted liquid is returned to the head of the treatment plant for re-treatment, since it still contains a large amount of suspended solids.
Concerning decantation of treated liquid, a variety of float pumps are known in the prior art (see for example U.S. Pat. No. 5,186,610). Such pumps are designed to float on a surface, such as a pond or lagoon, so that as liquid is pumped out, the pump itself will descend within the confines of the large area. These devices, however, are not structurally sufficiently durable and controllable with the confines of a small vessel used in sequential batch treatment of large quantities of water. The treatment process which requires additives to coagulate and flocculate, and there must be no obstructions below the surface where settling solid can cling and accumulate.
In addition to U.S. Pat. No. 5,358,644, a skimmer or decant mechanism is provided using a ridged pipe with flexible couplings at each end. The pipe extends downwardly and projects from the center of the vessel to the outer walls, and through the wall of the tank, so that when the valve at the lower end of the pipe is opened, liquid will flow by gravity from the decant downwardly through the pipe. The decanter then rides downwardly with the level of liquid to a predetermined depth. The obstruction below the surface creates an unwanted condition where settling solids can cling and accumulate. This also has the undesirable feature of requiring a pipe which extends downwardly through the tank on which the decanter rides.
U.S. Pat. No. 3,447,688 describes an adjustable tubular skimmer in a fixed location. The range of the adjustment is the diameter of the tube. It is intended for use in a continuous flow sewage plant, and therefore, is not pertinent to a sequential batch water treatment plant, wherein the separate phases of adding chemicals to effect coagulation, flocculation and settling are combined in a single vessel.